Method for forming thin films

ABSTRACT

The present invention provides a method for forming a film of aluminum oxide in which a target containing aluminum is sputtered in a gas containing fluorine atoms.  
     The thin film of aluminum oxide according to the present invention has little optical absorption and high refractive index in the ultraviolet and vacuum ultraviolet regions.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a method for forming thin films thatare used for optical parts as a reflection preventing film or reflectionenhancing film, and especially to a method for forming thin filmssuitable for use in optical parts applicable in ultraviolet and vacuumultraviolet regions because they have a good spectroscopiccharacteristics in the ultraviolet region (wavelength of 230 to 400 nm)and vacuum ultraviolet region (wavelength of 190 to 230 nm).

[0003] 2. Related Background Art

[0004] Methods for forming thin films by sputtering are widely used inthe conventional art because they are rather easily applicable forforming thin film of metals, insulators and various kind of compounds.While there are a variety of sputtering methods such as magnetronsputtering and facing sputtering:

[0005] In the magnetron sputtering, known as a sputter capable of a highspeed deposition, electrons are confined by magnetic field to increaseplasma density.

[0006] A method for forming a film of indium-tin mixed oxide (ITO) bysputtering in a gas containing fluorine together with hydrogen and wateris disclosed in Japanese Patent Publication No. 6-506266/InternationalPublication WO 92/17620.

[0007] In U.S. Pat. No. 4,125,446, a method for forming a metallic filmof aluminum by sputtering in a gas containing water and Ar is disclosed.

[0008] In a sputtering process for forming oxide films like alumina(Al₂O₃), alumina or aluminum (Al) is used as a target material and thinfilms are formed by a sputtering or reactive sputtering in a mixed gasof argon (Ar) and oxygen (O₂). In the methods for forming thin films bysputtering or reactive sputtering, a target material is ejected by ionsaccelerated under an ion-sheath voltage.

[0009] When the target material is composed of a compound, for examplean alumina (Al₂O₃) target, the sputtering particles of alumina ejectedfrom the target by an ion impact are decomposed and ejected from thetarget. The sputtering particles ejected are oxidized by colliding withoxygen or the like in the plasma or on the surface of substrates.

[0010] The method for forming a thin film of aluminum oxide is disclosedin Japanese Patent Application Laid-Open No. 7-70749.

[0011] An example of the film-forming apparatus is a sputtering system(an apparatus for forming sputtering thin films) in which an ion sourceis mounted for the purpose of enhancing the reactivity by an ion-assisteffect by irradiating the ions to the substrate.

[0012] A sputtering system and method for forming sputtering films areproposed in Japanese Patent Application Laid-Open Nos. 7-258841 and7-258845, wherein a positive voltage is applied on a target electrode toprevent abnormal electric discharge during sputtering. It was provedthat a SiOF film having a lower dielectric constant than that ofconventional silicon oxide (SiO₂) films is formed when a mixed materialof oxides with fluorides is used. For example, it was reported that SiOFfilms containing F atoms was formed by a plasma chemical vapordeposition (CVD) method by adding a mixed gas containing fluorine.

[0013] However, the alumina thin film formed by this method containsunreacted bonds (dangling bonds) that are bond deficiencies, therebyforming a film containing less numbers of oxygen atoms than thoserequired for satisfying a stoichiometric oxygen content.

[0014] Recently, so called eximer stepper using an eximer laser thatemits vacuum ultraviolet light having a short wavelength range (190 to230 nm) is used for a light source for a projection aligner forproducing semiconductor devices to attain a high resolution.

[0015] The thin films of alumina (Al₂O₃) formed by the conventionalmethod for forming thin sputtering films and system for forming the samedescribed above has a rather large absorption in the ultravioletwavelength region and, especially, in the vacuum ultraviolet wavelengthregion. Therefore, it is difficult to use these thin films as opticalthin films for ArF eximer steppers.

[0016] Twenty to thirty pieces of lenses are usually combined in theprojection optical system for producing semiconductor devices. It isimportant to form several to dozens of multi-film layers of dielectricmaterials, as reflection preventing layer, having different refractiveindices with each other on the surface of each lens to reduce thereflection index of the lens.

[0017] Suppose that 30 pieces of lenses are used in the projectionoptical system and the absorption of the reflection preventing film oneach lens is 1% in the working wavelength region of ultraviolet andvacuum ultraviolet light. Then, the intensity of the transmitted lightis reduced to 54.7% (the 60th power of 0.99) of the incident beam sincethe light transmittance decays in proportion to n-th power (n is thenumbers of the lens used) of transmittance. Moreover, when thereflection preventing film has some degree of light absorption, theabsorbed light energy is transferred into heat energy, which affect thequality of the printed images due to distortion of the lens. Thereflection preventing film at the site where the eximer laser beamfocuses may be broken.

SUMMARY OF THE INVENTION

[0018] An object of the present invention is to provide an optical thinfilm with little optical absorption and a good spectroscopiccharacteristics in the ultraviolet region and in the vacuum ultravioletregion.

[0019] An another object of the present invention is to provide anoptical thin film comprising aluminum oxide containing fluorine.

[0020] Still another object of the present invention is to provide anoptical film comprising aluminum oxide having the ratio of the number ofthe atoms other than aluminum to the number of aluminum atoms is largerthan 1.55 and smaller than 1.85.

[0021] Still further object of the present invention is to provide anoptical thin film comprising aluminum oxide prepared by sputtering atarget containing aluminum in a gas comprising fluorine atoms in which,if required, oxygen, water and helium are added.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a schematic illustration of the system for forming thinfilms according to the present invention.

[0023]FIG. 2 is a chart for describing the operation of the apparatusillustrated in FIG. 1.

[0024]FIG. 3 is a schematic illustration of one example of a thin filmforming system used in the present invention equipped with a fluorinemonitor.

[0025]FIG. 4 is a schematic illustration of another example of a thinfilm forming system used in the present invention.

[0026]FIG. 5 is a graph indicating the relation of the contents offluorine and hydroxyl groups in alumina with the optical absorptivity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0027]FIG. 1 is a schematic illustration of the main components of thesystem for forming sputtering thin films according to the presentinvention. In the figure, numeral 1 is a vacuum vessel as a reactionchamber. Mounted in the vacuum vessel 1 are an evacuation means 2 forevacuating the air in the vessel, a gas feed means 12 for introducingvarious gases into the vessel, and a shutter driving system 17 fordriving a shutter plate 16 equipped in the vessel. A cathode electrode 3and a substrate holder 5 for holding a substrate 6 at a position opposedto the cathode electrode 3 are disposed in the vacuum vessel 1. Analuminum target 4 that serves as a target material is attached to thecathode electrode 3. The substrate holder 5 has a device for heating thesubstrate 6 attached to it. The shutter plate 16 is disposed at the footof the substrate 6 to control the timing for forming thin films (thinfilms of alumina) on the substrate 6.

[0028] A high frequency power source 8 is connected to the cathodeelectrode 3 via a matching box 7 for matching discharge impedance. Oneterminal of a switch 10 is connected to the cathode electrode 3 via alow pass filter 9 for cutting high frequency components off from thehigh frequency power source 8. The other terminal of the switch 10 isconnected to a DC power source 11 for applying DC voltage equal to theself-bias potential of plasma.

[0029] Sputtering gases, oxygen (O₂) and NF₃ gas containing fluorine,are piped to the gas feed means 12 via mass flow controllers (MFC) 13and 14 for adjusting gas supply. The mass flow controller 14 is providedfor adjusting the supply of NF₃ gas containing fluorine while a controlsystem 15 is also provided for controlling the switch 10.

[0030] In the present embodiment, 200 sccm of oxygen gas (O₂) isintroduced into the vacuum vessel 1 via the mass flow controller 13 inthe gas inlet means 12 after sufficiently evacuating the air inside ofthe vacuum vessel 1 by the evacuation means 2. Argon gas may beincorporated in the oxygen gas. A plasma 18 is generated by supplying ahigh frequency electric power to the cathode electrode 3 and Al target 4from the high frequency power source 8 via the matching box 7. The highfrequency electric power is supplied up to a prescribed level whileadjusting the matching so that the intensity of reflected high frequencywave will be kept to its minimum value. After completing apre-sputtering step for cleaning the surface of the target 4 and forstabilizing discharge, the shutter plate 16 is turned open by thedriving force of the shutter driving system 17 to start forming thinfilms on the substrate 5.

[0031] The elements 9, 10, 11 and 15 described above form a means fortemporarily keeping the repeating self-bias near the earth potentialwhile the target material is subjected to a sputtering discharge atmetal materials (Al).

[0032] In the system for forming sputtering thin films according to thepresent embodiment, means for adding fluorine or gases of fluoridecompounds are used when sputtering thin films are formed. Fluorineatoms, fluoride ions and fluoride radicals are formed through a complexdecomposition reaction of fluorine or fluoride compounds added duringthe plasma discharge. The monovalent fluorine atoms, fluoride ions andfluorine radicals formed are so reactive that they are liable to beinvolved in bond terminations by reacting with the unreacted bonds(dangling bonds) of bond deficiencies in the thin film formed bysputtering.

[0033] When the self-bias during sputtering is temporarily adjusted tothe potential near the earth potential, sputtering is suppressed, thoughthe plasma is continuing, and few thin films are formed because ionacceleration to the surface of the target diminishes. When gassessupplemented with fluorine or fluoride compounds are introduced bysynchronizing with adjusting the self-bias close to the earth potential,they are decomposed in the plasma and fluorine atoms, fluoride ions andfluoride radicals formed react with unreacted bonds (dangling bonds) inbond deficiencies on the surface of the substrates, thereby terminatingthe chemical bonds.

[0034] By alternately repeating the steps for film formations and forreactions on the surface of the substrate, the density of the bonddeficiencies in the films formed can be decreased.

[0035] An excellent optical thin film having low absorption inultraviolet and vacuum ultraviolet regions and being durable toirradiation of ultraviolet or vacuum ultraviolet laser is obtained bythe process described above.

[0036]FIG. 2 is a time chart describing various operations in forming athin film of alumina on the substrate 5 according to the presentembodiment. The figure contains a time chart representing a highfrequency voltage having an amplitude of Vrf that is applied at thebeginning of electric discharge, at a pre-sputtering stage and at aninitial stage of thin film formation, a control signal for the switch10, and a gas supply (O₂ and NF₃ gases) configuration from the gas feedmeans. When a high frequency voltage having an amplitude of Vrf isapplied to the target 4 before the electric discharge starts, electronswith a small mass follow along with the electric field and generates aplasma by colliding with gaseous molecules having heavy masses.Electrons are accumulated on the surface of the target 4 due to adifference in mobility between electrons and ions, which impartsnegative vias (Vb) to the target.

[0037] After the pre-sputtering step has completed, formation of thinfilms on the substrate 5 starts by opening the shutter plate 16. Afterstarting to form thin films, a control signal is applied from thecontrol system 15 for 5 sec. to the switch 10 and the mass flowcontroller 14 for adjusting NF₃ gas feed. The switch 10 turns close for5 sec. by the control signal applied to the mass flow controller 14,thereby supplying fluoride gas. A DC voltage is supplied from the DCpower source 11. The prescribed output level of this DC voltage isadjusted to an approximately equal level with the self-bias voltage butits polarity is reversed. Electrons accumulated on the surface of thetarget 4 flows through the cathode electrode 3, low-pass filter 9 andswitch 10 to the DC power source 11 and the bias potential is made tothe level close to the earth potential. The plasma is still continuingbecause the high frequency voltage is being impressed on the target 4.

[0038] NF₃ gas with a prescribed flow rate is supplied in the vacuumvessel 1 for 5 sec. when a control signal synchronized with the signalfrom the switch 10 is supplied to the mass flow controller 14 foradjusting the supply of NF₃ gas. The NF₃ gas supplied is decomposed intohighly reactive monovalent fluorine atoms, fluorine radicals andfluoride ions in the plasma 18, which react with the unreacted bonds ofthe bond deficiencies in the thin films of alumina formed on thesubstrate 5. While nitrogen atoms and nitrogen radicals are also formedin the plasma 18, their reactivities are so low compared withoxygenation or fluorination reactions that a nitrogenation reaction withthe thin films of alumina formed on the substrate 5 does not start.

[0039] An excellent thin film with few bond deficiencies can be formedby alternately repeating the steps mainly for film formation and mainlyfor reactions.

[0040] The signal cycle of the control signals from the control system15 has a close relation with the film-forming rate. A signal cycle of0.1 Hz or less is preferable. Since the film-forming rate of the Al₂O₃thin film is 0.03 nm/sec when 2.5 kw of high frequency power is suppliedto the Al target 4 with an area of 5×15 square inches while the size ofthe alumina molecule is several tenth nm, the reaction process was setto start after forming a single layer in this preferred embodiment.

[0041] Because fluorine series of gases with high reactivities areintroduced in the film formation according to the present embodiment, itis preferable to use aluminum (Al) materials, which are resistive tofluorine and do not cause any problems when the materials migrate intothe film as contaminants, for the vacuum vessel 1 and for the componentsin the vacuum vessel 1.

[0042] The mass flow controller 14 for adjusting the supply of NF₃ gasfrom the gas feed means 12 should be disposed as close as possible tothe vacuum vessel 1 to minimize the delay time for introducing the gas.It is also preferable that signals are applied from the control system15 to the mass flow controller 14 by taking the delay time forintroducing the gas into account.

[0043] Although NF₃ gas was used as a gas containing fluoride compounds,F₂, SiF₄, CF₄, C₂F₂ or C₄F₈ may be used instead of NF₃ gas. These gaseswere selected from those satisfying the conditions below.

[0044] (1-a) The gases should contain fluorine or fluoride compoundsbecause the unreacted chemical bonds (dangling bonds) in the bonddeficiencies are terminated by bonding with fluorine atoms.

[0045] (1-b) In the case of gases of fluoride compounds, the absorptionedge of the compounds formed by oxidizing the atoms bonded to fluorineatom should be in the vacuum ultraviolet region.

[0046] (1-c) In the case of gases of fluoride compounds, the atomsbonding to fluorine atom should have low reactivities than oxidationreactions or fluorination reactions.

[0047] In the process for forming sputtering thin films described above,the amount of fluorine or gases of fluoride compounds added ispreferably from 0.5% to 20% of the amount of the sputtering gas. Whenthe amount is 0.5% or more, the gas exhibits a termination effect thatallows the gas to react with the unreacted chemical bonds (danglingbonds) of the bond deficiencies in the thin film formed, thereby makingit possible to obtain a film without any optical absorption in thewavelength range from the ultraviolet region having a wavelength of 300nm or below to the vacuum ultraviolet region having a wavelength of 193nm.

[0048] When the amount of fluorine or gases of fluoride compounds addedis more than 20%, some troubles as described below are seen in theprocess for forming thin films of alumina.

[0049] (2-a) When the film-forming rate of aluminum fluoride (AlF₃) isvery rapid and the amount of the gases added is large during pulse-wiseintroduction of the reaction gases, a large amount of aluminum fluoride(AlF₃) is formed in the sputtering thin film of alumina by the effect ofresidual fluorine. Aluminum fluoride (AlF₃) has an inherentdeliquescence that allows the material to be dissolved out by absorbingmoisture in the air, thereby causing environmental problems as well asdeteriorating durability.

[0050] (2-b) Both of the materials having a high refractive indices andlow refractive indices are required for preparing a film having suchoptical characteristics as reflection preventing films in the regionsfrom the ultraviolet wavelength of 300 nm or below to the vacuumultraviolet wavelength of 193 nm. However, any optical materials havinga high refractive indices suitable for use in the wavelength regiondescribed above are not available today. Therefore, the required opticalcharacteristics are only attained by using alumina (Al₂O₃) that is anintermediate refraction material having a refractive index of n=1.85 at193 nm. For example, a material with excellent optical characteristicssuitable for forming a reflection preventing film in a broad wavelengthrange can be obtained by a combination of the materials having a largedifference between an intermediate refractive index and a low refractiveindex. Since aluminum fluoride (AlF₃) has a low refractive index ofn=1.45 at a wavelength of 193 nm, the refractive index (n) of thealumina film becomes 1.77 or less when the content of fluoride afterforming the sputtering thin film of alumina exceeds 20% by weight. Thedifference between the refractive index of alumina thus formed and thatof the material with a low refractive index—SiO₂, MgF₂ or CaF₂—is sosmall that it is difficult to obtain an excellent opticalcharacteristics.

[0051] In the present embodiment, optical parts like lenses and mirrors,on which a thin film of alumina obtained by using the method and systemfor forming sputtering thin films described above is applied, are usedfor optical projection systems for effectively producing semiconductordevices.

[0052] According to the embodiments of the present invention, asputtering thin film which is suitable for optical systems used in theultraviolet and vacuum ultraviolet regions can be produced byappropriately selecting each element of the system and steps for forminga thin film on the substrate, especially by film-forming an alumina (Al₂0 ₃) thin film on the substrate. This film has little absorption in theultraviolet and vacuum ultraviolet regions and other characteristicsrequired for use described above.

[0053] Thin films of alumina formed by the conventional sputteringmethods or reactive sputtering methods have unreacted bonds (danglingbonds) due to the bond deficiencies in the film. A high absorption ofthe film in the ultraviolet and vacuum ultraviolet regions is ascribedto the presence of these deficiencies.

[0054] According to the present embodiment, on the other hand, fluorineor gases of fluoride compounds are added by synchronizing with theself-bias control of the target during the process for forming thinfilms of alumina when unreacted bonds (dangling bonds) due to bonddeficiencies are liable to be formed. The process is divided intoalternately repeating two steps of mainly forming the films and mainlyallowing to react the gases on the surface of the substrate, therebyreducing the density of the bond deficiencies in the film formed andobtaining thin films with little optical absorption in the ultravioletregion and vacuum ultraviolet region.

[0055] The thin films of alumina with low absorption obtained by theembodiments of the present invention have a resistivity againstirradiation by a ultraviolet or vacuum ultraviolet lasers like KrF orArF eximer lasers. Further, the films have high refractive indices whenapplied on the optical parts to be used in the ultraviolet region (300nm or below) and vacuum ultraviolet region. When the film is combinedwith the materials having low refractive indices such as SiO₂, MgF₂ andCaF₂ that are suitable for use in the wavelength region described above,optical elements having low reflective indices in a wide wavelengthregion can be easily prepared.

[0056]FIG. 3 shows a system for forming thin films according to theanother embodiment of the present invention. This is a system by whichthe amount of fluorine is monitored during the film-forming process,provided with: a film-forming chamber 1 that is evacuated with a vacuumpump as an evacuation means 2 connected to an exhaust port, a substrateholder 5 as holding means disposed in the chamber, a rotary target unit3 as generator means for film-forming particles opposed to the substrateholder 5, power sources 8 and 11 for supplying electric current to thetarget, reaction gas feed means provided with reaction gas feed lines 13a, 14 a and 19 a for introducing the reaction gases into thefilm-forming chamber 1 via the mass flow controllers 13, 14 and 19, amass spectrometer 21 as a detection means for fluorine in the atmosphereof the film-forming chamber 1 and CPU 15 as control means connected tothe mass spectrometer. This CPU controls a heater 20 for heating theinside of the film-forming chamber 1, the mass flow controllers 13, 14and 19 in each reaction gas feed line 13 a, 14 a and 19 a and powersources 8 and 11.

[0057] The substrate holder 5 has a heater 5 a for heating a substrate 6as a base body mounted thereon, and rotates around an axis O₁ by arotary driving means (not shown). The rotary target unit 3 has a targetholder freely rotating around an axis O₂ perpendicular to the axis O₁ ofthe substrate holder 5, and one pair of target 4 held thereon. Bothtargets 4 are provided with magnets 4 a, on their opposed faces. Whenalternating multi-layers are formed by using two kind of film-formingmaterials, for example, each target of 4 is used by turns by rotatingthe target holder 3, thereby making it possible to continuously formeach layer without exposing the film-forming chamber 1 to the open air.

[0058] The concentration of generated fluorine in the film-formingchamber 1 is detected as fluoride ion current value with the massspectrometer 21 and the reaction gas feed lines are controlled bysending the signal from the mass spectrometer to CPU.

[0059] The amount of fluorine in the film-forming chamber 1 ispreviously determined by an experiment and the measured values arestored in CPU 15. CPU 15 controls the reaction gas feed lines bycomparing the detected values from the mass spectrometer 21 with thestored data in CPU 15.

[0060] According to the present embodiment shown in FIG. 3, a thin filmhaving a refractive index identical with the designed values can beconstantly formed by adjusting the fluorine levels in the film-formingchamber to a prescribed level. This greatly contributes to theimprovement of qualities of the optical thin films and growth ofproductivity.

[0061] In forming conventional thin films to be used in the ultravioletregion, H₂O was introduced in addition to sputtering gases and reactiongases as disclosed in Japanese Patent Application Laid-Open No.07-070749. Any optical absorption against the KrF laser beam having awavelength of around 248 nm was not observed in the films. When the samemethod for forming thin films was applied for the optical parts to beused for the optical systems for ArF eximer laser, the film showed anabsorption at a wavelength of 193 nm.

[0062] In thin films of light-transmissive oxides commonly used foroptical systems, optical absorption becomes more dominant within thethin films as the wavelength becomes shorter; i.e., the ratio of theintensity of the transmitted light against the incident beamintensity—optical transmittance—decreases as the wavelength isdecreased. For these reasons, it is supposed that the conventionaltechniques for forming thin films could not comply with the requirementto reduce the optical absorption when ArF laser having a more shorterwavelength region around 193 nm than that of KrF laser is used.

[0063] This optical absorption prevents the transmittance of the filmfrom being increased, which has been the original object according tothe present invention. Moreover, the absorbed light energy istransferred within the thin film into heat energy that causes atemperature increase of substrate, or optical glass elements, thatresults in a functional deterioration of the elements. Therefore, it isdesirable to reduce the absorption of the thin film as small aspossible.

[0064] From the view point of micro-structures of the film, bonddeficiencies in the film is mainly responsible for the increase in lightabsorption. The bond deficiencies are related to the presence of atomsin the thin film having unreacted bonds (dangling bonds) that induceoptical absorption. Supplying fluorine atoms or hydroxyl groups (OH) tothe deficiency sites for allowing them to react with the unreacted bondsis effective for quenching the bond deficiencies.

[0065] An object of the examples described hereinafter is, by quenchingthe unreacted bond, to provide a thin film without any absorption evenat short wavelength region and a method for forming the same.

[0066] The method according to the examples to be described hereinaftercomprise a step of introducing at least one of the gases of CF₄ and NF₃,and H₂O at least simultaneously into the reaction vessel to deposit thinfilms. In this method, CF₄ gas and NF₃ gas, and H₂O are decomposed inthe plasma forming F (radicals or ions) and OH (radicals or ions),respectively, which react with unreacted bonds in the thin films toquench the bond deficiencies responsible for optical absorptions.

[0067] Especially, in the process for forming alumina films, the massflow rate during the film-formation is adjusted so that the content offluorine in the thin film becomes 2.0% by weight or more, morepreferably 2.0 to 20% by weight, for the purpose of further reducing theoptical absorption of the film.

[0068] Furthermore, in the process for forming alumina films, the massflow rate during the film-formation is adjusted so that the content ofhydroxyl group in the thin film becomes 0.5% by weight or more for thepurpose of further reducing the optical absorption of the film.

[0069] Accordingly, when the mass flow rate during the film formation isadjusted so that the contents of fluorine and hydroxyl group in the thinfilm become 2.0% or more and 0.5% by weight or more, respectively, analumina film free from optical absorption can be prepared.

[0070] The apparatus for forming optical thin films used in the examplesbelow is described referring to FIG. 4.

[0071] A substrate holder 5 for holding the substrate 6 and targetholder 3 for holding the target 4 are provided in the film-formingchamber 1. The film-forming chamber 1 is connected to a vacuum pump 2through an exhaust port. An RF voltage having a frequency of 13.56 MHzis impressed on the target 4 from the high frequency power source 8.

[0072]101 is a sputtering gas feed means in which valves and mass flowcontrollers are provided from which Ar or He, or a mixed gas of Ar andHe is supplied. Similarly, 102 is a oxygen feed means and 103 and 104are feed means of the gases containing fluorine atoms. In this figure,103 provides a feed means for fluorocarbons while 104 provides a feedmeans for nitrogen fluoride.

[0073]105 is a feed means for water, which supplies water vaporevaporated at an isothermal bath 106 into the film-forming chamber 1through the mass flow controllers and valves.

EXAMPLE 1

[0074] The effect of simultaneously introducing H₂O and CF₄ will bedescribed in the example of the film forming method by sputtering.

[0075] The system used in this example was that illustrated in FIG. 4.An aluminum target was placed on a holder 3 to which a high frequencyelectric power can be impressed and on the back face of which a magnetwas attached. A substrate 6 on which thin layers were to be laminatedwas disposed in a vacuum vessel 1. Ar, O₂, CF₄ and NF₃ gases and H₂Ovapor were separately introduced into the vessel 1 through each gas feedmeans. H₂O was introduced after evaporating the liquid to a vapor in anisothermal bath 7.

[0076] In this example, H₂O vapor and CF₄ gas were introduced in thevacuum vessel at mass flow rates of 2.0 sccm and 3.5 sccm, respectively,while feeding Ar and O₂ gases at mass flow rates of 20 sccm and 80 sccm,respectively, thereby depositing an alumina film of about 100 nm inthickness on a quartz substrate.

EXAMPLE 2

[0077] H₂O vapor and CF₄ gas were introduced in a vacuum vessel at acombined three different mass flow rates while feeding O₂ at a mass flowrate of 100 sccm by using the same system as used in Example 1 shown inFIG. 4, thereby depositing an alumina film of about 100 nm in thicknesson a quartz substrate followed by measurements of optical absorptivity.The results are shown in Table 1.

[0078] The optical absorptivity was calculated as(100%−transmittance−reflectance) where transmittance and reflectancewere measured by a spectrophotometer. The measuring wavelength was 193nm which is the wavelength emitted from ArF laser. TABLE 1 Mass FlowRate (sccm) Optical H₂O CF₄ absorptivity (%) Sample 1 0 0 8.6 Sample 22.0 0 2.4 Sample 3 2.0 3.5 0.0

[0079] By comparing this result with that of Example 1, it was shownthat the optical absorptivity was smaller in this example where Ar wasnot introduced and the alumina film which was formed by introducing bothof H₂O vapor and CF₄ gas has also smaller optical absorptivity.

EXAMPLE 3

[0080] An alumina film was formed by varying the feed mass of H₂O vaporand CF₄ gas with a constant mass flow rate of oxygen of 100 sccm byusing the same apparatus as used in Example 1 shown in FIG. 4. Therelation of the ratio of the hydroxyl group and fluorine contents in thealumina film to the total weight of the film with the opticalabsorptivity of the film was investigated.

[0081] The optical absorptivity was calculated as(100%−transmittance−reflectance) where transmittance and reflectancewere measured by a spectrophotometer. The measuring wavelength was 193nm which is the wavelength emitted from ArF laser. The contents ofhydroxyl group and fluorine in the film were determined by ESCA.

[0082] When the proportion of hydroxyl group in the film was 0.5% byweight or more, optical absorption was rapidly decreased while, when theproportion of fluorine was 2.0% by weight or more, the opticalabsorption showed a tendency to be reduced. When the proportions ofhydroxyl group and of fluorine were 0.5% by weight or more and 2.0% byweight or more, respectively, an optical absorption of as small as 0.2%or less was realized.

[0083] These results indicate that an oxide thin film with a smalloptical absorption containing both of fluorine and hydroxyl group can beformed by at least simultaneously introducing at least one of the gasesof CF₄ and NF₃, and H₂O vapor.

EXAMPLE 4

[0084] The gases of CF₄ and NF₃ introduced have an effect for reducingthe optical absorption of the film because F atoms generated by adecomposition of these gases in a plasma during sputtering react withunreacted bonds (tangling bonds) at the sites of bond deficiencies inthe thin film. It is sometimes observed, however, that an excess amountof F atoms incorporated into the thin film also reduce the refractiveindex of the film.

[0085] Although the changes in refractive indices should not always benoticed, a design of the optical films for realizing desired opticalcharacteristics would be difficult when the refractive index of the filmhas been reduced. Despite a reduction of the contents of CF₄ and NF₃ inthe film is desirable in one viewpoint, an excessive reduction of themwould make it hopeless to decrease the optical absorption.

[0086] The examples to be described hereinafter is to provide a thinfilm that do not have any absorption and have a high refractive index inthe ultraviolet region.

[0087] Thin films were allowed to deposit by introducing oxygen, H₂Ovapor, at least one of the gases of CF₄ and NF₃, and He in the vacuumvessel when the films were formed by the sputtering method according tothe examples described hereinafter.

[0088] The effect of introducing He is that helium atoms are excited inthe plasma and are converted to metastable atoms followed by generatingactivated oxygen by reacting with oxygen molecule, thereby curinginsufficient oxidation of the film to decrease the optical absorption.On the other hand, no reduction in the refractive index is observedcompared with the cases when CF₄ and NF₃ gases are introduced withoutintroducing He gas. However, since the problem of reducing the opticalabsorption in the ultraviolet region can not be fully settled only byintroducing O₂, H₂O and He, introduction of CF₄ and NF₃ gases togetherwith the above gases should be considered.

[0089] After all, in order not to lower the refractive index of the filmwithout increasing the optical absorption of the thin film, it is morepreferable that the amount of introduction of three kind of gases of O₂,H₂O and He is at first optimized so that the optical absorption isminimized, then optimizing the amount of introduction of CF₄ and NF₃gases again to minimize the optical absorption of the film.

[0090] Further decrease in the optical absorption of the film is madepossible when the mass flow rates of the gases during the formation ofaluminum oxide films are adjusted so that 0.5% by weight or more ofhydroxyl group and 2.0% by weight or more of fluorine are contained inthe thin film.

[0091] The system illustrated in FIG. 4 was again used in this Example4. Sputtering was carried out by introducing He, O₂, CF₄ and H₂O in thevacuum vessel in which an aluminum target was placed on the targetholder 3.

[0092] Oxygen was fed at a mass flow rate of 100 sccm while H₂O, He andNF₃ gases were introduced at each combination of mass flow rate aslisted in Table 2 in the vacuum vessel. A film of Al₂O₃ with a thicknessof 100 nm was deposited in each experiment by impressing 500 W of highfrequency power on the target. The results of measurements of opticalabsorptions and refractive indices are also shown in Table 2.

[0093] The optical absorptivity was calculated as(100%−transmittance−reflectance) where transmittance and reflectancewere measured by a spectrophotometer. The measuring wavelength was 248nm and 193 nm which are the wavelength emitted from KrF and ArF lasers,respectively. The refractive index was measured at 193 nm which is thewavelength emitted from ArF laser. TABLE 2 Mass flow rate Optical (sccm)absorptivity (%) Refractive H₂O He CF₄ 248 (nm) 193 (nm) index Sample 42.0  0 0 0.3 2.5 1.83 Sample 5 2.0  0 1.5 0.0 0.0 1.76 Sample 6 2.0 2500 0.0 1.5 1.82 Sample 7 2.0 250 0.7 0.0 0.0 1.81

[0094] By comparing Sample 4 with Sample 6, it is shown that, when Hewas introduced, the optical absorption of Sample 6 was reduced but notfully extinguished although little reduction in the refractive index wasobserved. In Sample 5, the optical absorption was reduced to 0% byintroducing a large amount of CF₄ but the refractive index was alsoconsiderably reduced. When the mass flow rate of CF₄ was adjusted whilekeeping the mass flow rates of H₂O and He in Sample 6 constant so thatthe optical absorption becomes 0%, a film of Sample 7 in which therefractive index was not so largely reduced as compared with that ofSample 4 was obtained.

[0095] The results above indicate that Sample 7 formed by optimizing themass flow rates of H₂O, He and CF₄ gives a film having a high refractiveindex without any optical absorption at 248 nm and 193 nm.

EXAMPLE 5

[0096] A film of Al₂O₃ was formed by keeping a mass flow rate of O₂ at100 sccm with changing the mass flow rates of H₂O and CF₄ by using thesame apparatus as was used in Example 1. The relation between the weightratio of hydroxyl group and fluorine to the total weight of the film andthe optical absorptivity is shown in FIG. 5.

[0097] The optical absorptivity was calculated as(100%−transmittance−reflectance) where transmittance and reflectancewere measured by a spectrophotometer. The measuring wavelength was 193nm which is the wavelength emitted from ArF laser. The contents ofhydroxyl group and fluorine in the Al₂O₃ film were measured by ESCA.

[0098] The figure shows that the optical absorption rapidly decreaseswhen the contents of hydroxyl group and fluorine are 0.5% by weight ormore and 2.0% by weight or more, respectively. The content of fluorinewas never increased up to 6.0% by weight or more even when its mass flowrate was increased. Therefore, it was made clear that an opticalabsorption as small as 0.1% or less can be realized in a range that thecontents of hydroxyl group is 0.5% by weight or more and that offluorine is 2.0% by weight or more.

[0099] This example indicates that an oxide film with little opticalabsorption and high refractive index can be formed by introducing atleast one of fluorine-containing gases such as the CF₄ gas or NF₃ gastogether with H₂O and He when the thin film is formed by a sputteringmethod.

EXAMPLE 6

[0100] A stoichiometric ratio of Al:O=2:3 has been recognized to beoptimal for reducing the optical absorption when optical thin films ofAl₂O₃ for uses in the ultraviolet wavelength region are formed.

[0101] However, in the Al₂O₃ films formed based on the above technicalconcept, the optical absorption was not reduced at a wavelength of ArFeximer laser (193 nm).

[0102] Therefore, gases containing H compounds and F compounds and He inaddition to conventional gases containing O₂ were introduced into thevacuum vessel of the sputtering system for the purpose of solving theproblems described above in forming optical thin films of Al₂O₃. Thebinding ratio of Al to O in the thin film formed by this method is not2:3 but O is excessively bound to Al. The film also contains OH groupsthat are recognized to terminate excess bonds. This is the condition forreducing the optical absorption and for providing a thin film having alow optical absorption at a wavelength of ArF eximer laser (193 nm).

[0103] In this example, sample 8 was prepared by using the system shownin FIG. 4.

[0104] Al and quartz were used as a target and substrate, respectively,and a RF power source was used. The method for forming optical thinfilms according to the present example is as follows:

[0105] The inside of the vacuum chamber was first evacuated to 1×10⁻⁴ Paor less. Then, O₂ gas, He gas and H₂O gas were introduced at mass flowrates of about 100 sccm, 100 sccm and 2 sccm, respectively. An electricpower from the power source was supplied to the target to generate aplasma. The target component Al was sputtered by this plasma and reactedwith oxygen to form a thin film of aluminum oxide. The atomic ratio ofAl and O in the film of aluminum oxide formed by this method wasconfirmed to be 1:1.7 by an assay using ESCA. The proportion of oxygenthat was bound to Al to the total oxygen in the film was 96.4% while theproportion of oxygen incorporated into OH bonds was 3.6%. The opticalabsorptivity of the film formed by this method at a wavelength of ArFeximer laser (193 nm) was only 0.6% per 100 nm of film thickness.

[0106] Sample 9 was prepared as follows:

[0107] Al and quartz were used as a target and substrate, respectively,and a RF power source was used.

[0108] The inside of the vacuum chamber was first evacuated to 1×10⁻⁴ Paor less. Then, O₂ gas, He gas and H₂O gas were introduced at mass flowrates of about 100 sccm, 200 sccm and 2 sccm, respectively. An electricpower from the power source was supplied to the target to generate aplasma. The target component Al was spattered by this plasma and reactedwith oxygen to form a thin film of aluminum oxide. The atomic ratio ofAl and O in the film formed by this method was confirmed to be 1:1.71 byan assay using ESCA. The proportion of oxygen that is bound to Al to thetotal oxygen in the film was 96.9% while the proportion of oxygenincorporated into OH bonds was 3.1%.

[0109] The optical absorptivity of the film formed by this method at awavelength of ArF eximer laser (193 nm) was only 0.3% per 100 nm of filmthickness.

[0110] Sample 10 was prepared as follows:

[0111] Al and quartz were used as a target and substrate, respectively,and a RF power source was used.

[0112] The inside of the vacuum chamber was first evacuated to 1×10⁻⁴ Paor less. Then, O₂ gas, CF₄ gas and H₂O gas were introduced at mass flowrates of about 100 sccm, 3.5 sccm and 2 sccm, respectively. An electricpower from the power source was supplied to the target to generate aplasma. The target component Al was spattered by this plasma and reactedwith oxygen to form a thin film of aluminum oxide. The atomic ratio ofAl, O and F in the aluminum oxide film formed by this method wasconfirmed to be 1:1.59:0.21 by an assay using ESCA. The proportion ofoxygen that was bound to Al to the total oxygen in the film was 97.3%while the proportion of oxygen incorporated into OH bonds was 2.7%. F inthe film was not bound to Al.

[0113] The optical absorptivity of the film formed by this method at awavelength of ArF eximer laser (193 nm) was only 0.1% per 100 nm of filmthickness.

[0114] Sample 11 was prepared as follows:

[0115] Al and quartz were used as a target and substrate, respectively,and a RF power source was used.

[0116] The inside of the vacuum chamber was first evacuated to 1×10⁻⁴ Paor less. Then, O₂ gas, He gas and H₂ gas were introduced at mass flowrates of about 180 sccm, 200 sccm and 30 sccm, respectively. An electricpower from the power source was supplied to the target to generate aplasma. The target component Al was spattered by this plasma and reactedwith oxygen to form a thin film of aluminum oxide. The atomic ratio ofAl and O in the film formed by this method was confirmed to be 1:1.65 byan assay using ESCA. The proportion of oxygen that was bound to Al tothe total oxygen in the film was 97% while the proportion of oxygenincorporated into OH bonds was 3%.

[0117] The optical absorptivity of the film formed by this method at awavelength of ArF eximer laser (193 nm) was only 0.3% per 100 nm of filmthickness.

[0118] The results obtained in Example 6 described above are listed inTable 3. The example allowed to form an optical thin film mainlycontaining Al₂O₃ having an optical absorptivity of 1% or less at awavelength of ArF eximer laser (193 nm) at a low temperature. TABLE 3Ratio of O Ratio of O Content of O bound bound Content of F (ratio toAl) to Al (%) to OH (%) (ratio to Al) Sample 8 1.70 96.4 3.6 0.0 Sample9 1.71 96.9 3.1 0.0 Sample 10 1.59 97.3 2.7 0.21 Sample 11 1.65 97.0 3.00.0

[0119] As described in the examples above, when the ratio of NE to NA,where the number of aluminum atoms is NA and that of atoms other than Alis NE, is more than 1.55 and less than 1.85, the thin films exhibitexcellent optical characteristics.

[0120] It is desirable that the ratio of NO to NA, where NO is thenumber of oxygen atoms, is more than 1.55 and less than 1.75.

[0121] It is further preferable that the proportion of the number ofoxygen atoms bound to aluminum atoms is 95% or more and that of thenumber of oxygen atoms bound to hydrogen atoms is 2.0% or more.

What is claimed is:
 1. A method for forming a film of aluminum oxide,comprising sputtering a target containing aluminum in a gas containingfluorine atoms.
 2. The method for forming a film of aluminum oxideaccording to claim 1 wherein the gas contains oxygen.
 3. The method forforming a film of aluminum oxide according to claim 1 wherein thecontent of fluorine gas added in the gas is 0.2% to 20%.
 4. The methodfor forming a film of aluminum oxide according to claim 3 wherein thegas is at least a gas selected from the group consisting of F₂, NF₃,SiF₄, CF₄C₂F₂ and C₄F₈.
 5. The method for forming a film of aluminumoxide according to claim 1 wherein the gas contains at least one of CF₄and NF₃, and H₂O.
 6. The method for forming a film of aluminum oxideaccording to claim 5 wherein the gas further contains oxygen and helium.7. The method for forming a film of aluminum oxide according to claim 1wherein the gas contains at least one of the gases of CF₄ and NF₃, andH₂O, O₂ and He.
 8. The method for forming a film of aluminum oxideaccording to claim 1 wherein the film of aluminum oxide is formed whilemonitoring the content of fluorine in the gas.
 9. The method for forminga film of aluminum oxide according to claim 1 wherein the aluminum oxidefilm is formed on a light-transmissive insulating substrate.
 10. Themethod for forming a film of aluminum oxide according to claim 9 whereinthe light-transmissive insulating substrate is quartz or fluorite.
 11. Afilm of aluminum oxide comprising fluorine.
 12. The film of aluminumoxide according to claim 11 wherein the content of the fluorine is 2.0%by weight or more.
 13. The film of aluminum oxide according to claim 11wherein the content of hydroxyl groups is 0.5% by weight or more. 14.The film of aluminum oxide according to claim 11 wherein the content ofthe fluorine is 2.0% by weight or more and the content of hydroxylgroups is 0.5% by weight or more.
 15. Optical parts in which a film ofaluminum oxide is formed on a light-transmissive insulating substrate,wherein the aluminum oxide film is the aluminum oxide film according toclaim 11 .
 16. A film of aluminum oxide having the ratio of NE to NA inthe aluminum oxide film wherein the ratio satisfies the followingrelation: 1.55<NE/NA<1.85 where NA is the number of aluminum atoms andNE is the number of the atoms other than aluminum atoms contained in thefilm.
 17. A film of aluminum oxide according to claim 16 wherein theratio of NO to NA in the aluminum oxide film wherein the ratio satisfiesthe following relation: 1.55<NO/NA<1.75 where NA is the number ofaluminum atoms and NO is the number of oxygen atoms contained in thefilm.
 18. A film of aluminum oxide according to claim 17 wherein theproportion of oxygen atoms bound to aluminum atoms is 95% or more andthe proportion of oxygen atoms bound to hydrogen atoms is 2.0% or morein the film.
 19. Optical parts in which a film of aluminum oxide isformed on a light-transmissive insulating substrate, wherein the film ofaluminum oxide is the aluminum oxide film according to claim 16 .
 20. Amethod for forming a sputtering thin film in which a plasma is generatedby applying a high frequency voltage to a cathode electrode in a vacuumvessel, ions in the plasma are accelerated and collided to a target tomake aluminum or alumina to be ejected from the target and a thin filmof alumina is formed on the substrate, comprising forming the thin filmof alumina by adding fluorine or a gas containing fluoride compoundsinto a sputtering gas.
 21. The method for forming a sputtering thin filmaccording to claim 20 wherein the sputtering gas is argon and/or oxygen.22. The method for forming a sputtering thin film according to claim 20wherein the amount of the gas containing fluorine or fluoride compoundsis within 0.5 to 20% of the sputtering gas.
 23. The method for forming asputtering thin film according to claim 20 wherein the fluoridecompounds are one or a plurality of the compounds selected from F₂, NF₃,SiF₄, CF₄, C₂F₂ or C₄F₈.
 24. The method for forming a sputtering thinfilm according to claim 20 wherein the thin film of alumina containsfluorine.
 25. The method for forming a sputtering thin film according toclaim 20 wherein means in which repeating self-vias is temporarily setnear the earth potential during sputtering discharges is used and a gassupplemented with fluorine or fluoride compounds is introduced bysynchronizing with a self-vias near the earth potential.
 26. The methodfor forming a sputtering thin film according to claim 25 wherein meansin which self-vias potential is set near the earth potential duringsputtering discharges is used and the repeating frequency is 0.1 Hz orless.
 27. A system for forming a sputtering thin film in which a plasmais generated by applying a high frequency voltage from a high frequencypower source to a cathode electrode provided in a vacuum vessel, ions inthe plasma are accelerated and collided to a target provided in thevacuum vessel to make aluminum or alumina to be ejected from the targetand a thin film of alumina is formed on the substrate, the thin film ofalumina being formed by adding fluorine or gases containing fluoridecompounds into a sputtering gas.
 28. The system for forming a sputteringthin film according to claim 27 wherein the sputtering gas is argonand/or oxygen.
 29. The system for forming a sputtering thin filmaccording to claim 27 wherein the amount of the gas containing fluorineor fluoride compounds is within 0.5 to 20% of the sputtering gas. 30.The system for forming a sputtering thin film according to claim 27wherein the fluoride compounds are one or a plurality of compoundsselected from F₂, NF₃, SiF₄, CF₄, C₂F₂ or C₄F₈.
 31. The system forforming a sputtering thin film according to claim 27 wherein means inwhich repeating self-vias is temporarily set near the earth potentialduring sputtering discharges is used and a gas supplemented withfluorine or fluoride compounds is introduced by synchronizing with aself-vias near the earth potential.
 32. The system for forming asputtering thin film according to claim 31 wherein means in whichself-vias potential is set near the earth potential during sputteringdischarges is used and the repeating frequency is 0.1 Hz or less.